Well, you have to have a sense of the pattern - should it be increasing? Decreasing? Staying the same? Are there matching decreases for the specified YOs?

I would also quibble about using the term YO in describing seed stitch, since you're not YOing there - you're bringing the yarn forward between the needles and not creating another stitch on your needles.

Personally, I would hope everything was documented, but obviously - as we have seen - sometimes it's not.

A YO is making a deliberate hole in the knitting, like lace, and should have decreases to offset them unless you're supposed to increase stitches. To switch yarn from front to back for knits and purls you move it between the needle tips, not over the needle.

Some patterns do have you bring yf to slip a stitch, then yb for the next stitch as part of the design. British terminology has 3-4 terms for a YO including yf/yfwd which is done between 2 knit stitches.

If it doesn't specify for a yo, then there is no yo. Bringing your yarn forward from a knit to a purl is something that is simply an inherent part of knitting and is never specifically called for. (The exception being if it's a design element like suzeeq mentioned.) If a pattern told you to bring your yarn forward or back every time you switched from knit to purl or vice versa, the pattern would be impossible to read for all the superfluous notations. That being said, if you think there is an actual yo that is not written into the pattern, you can Google the pattern or check Ravelry and see if there is an errata for the pattern.